The present invention pertains generally to prostheses. More specifically, the present invention pertains to prostheses which act as part of an elbow joint in a patient for motions which mimic anatomical movements. The present invention is particularly, but not exclusively, useful as a prosthesis for the radial head in the elbow joint of a patient.
Like other joints and anatomical features of the human body, the elbow joint is exceedingly complex in its make-up and function. Also like the other joints and anatomical features of the human body, the elbow joint is unique unto itself and requires specific consideration for its reconstruction or replacement. The complexity and uniqueness of this joint are quite interesting and are, perhaps, best appreciated by considering the skeletal motions which are involved in its movement.
In the transition of the hand and forearm from pronation to supination the radius and ulna of the forearm transition from a crossed relationship to a side-by-side relationship. Thus, in this movement there is a relative rotation of the radius bone about the ulna. Also, but more subtly, during the transition between pronation and supination there is also some relative translational movement between the radius bone and the ulna. The consequence of all this is that from a reference point on the ulna, the radius bone appears to move with a general motion that includes both translation and rotation. The head of the radius interacts with the capitellum and the radial notch of the ulna during pronation and supination, providing elbow and forearm stability during rotation and translation.
In addition to its importance as a component of forearm function, the radial head is an equally important component of normal elbow function. Indeed, elbow function involves bending, lifting and twisting movements, all of which require joint stability. Because motions in the human body require the interaction of various anatomical components, it is crucial that replacement of a component be precise in form, size, and orientation. While the head of the radius bone directly engages the capitellum of the humerus and the radial notch of the ulna, it also relates indirectly to other anatomical components of the arm. Specifically, ligaments surrounding the radial head are essential to elbow stability. Further, misalignment of the radius bone will cause poor radial-capitellar joint contact, leading to subluxation, or poor alignment of the elbow. It follows that the wrist and shoulder joints are also affected by the alignment of the radius bone.
The importance of having a workable prosthesis for the head of the radius bone is underscored by the debilitating effects which commonly result when a joint becomes damaged due to fracture, arthritis, or osteochondrosis. It is well known that radial head resection, as seen in elbow injuries, results in persistent elbow instability. Additionally, forearm axial instability can result from radial head excision if the remaining stabilizers, the supporting ligaments, are compromised. Because this loss of stability affects the interdependent functions of the elbow and forearm, when the radial head is damaged, it is common to see further damage to other components of the radial ulnar joint system, including, but not limited to, the complex system of supporting ligaments that encase the elbow joint. It has been well demonstrated that damage of any one of the components of the radial ulnar joint system leads to pain, weakness, and loss of motion. It is, therefore, of great importance to the patient that damage to the radial head be remedied.
In light of the above, it is an object of the present invention to provide a prosthesis for engaging a radius bone in the forearm of a patient with the capitellum of a humerus bone and the radial notch of the ulna at the elbow of a patient. It is a further object of the present invention to provide a prosthesis that is shaped to establish a secure engagement with the radius bone and is capable of cooperating with the anatomic structure and function of the portion of the radius being replaced. Yet another object of the present invention is to provide a system for implanting the prosthesis which places the forearm in the precise anatomical orientation necessary for implantation of the prosthesis. It is also an object of the present invention to precisely measure the length of the radial bone to be resected, and to precisely measure the size of the prosthesis to be implanted in place of the resected bone. Another object of the present invention to provide a prosthesis that is relatively easy to manufacture, is simple to implant, and is comparatively cost effective.
In accordance with the present invention, a prosthesis for replacing the head of a radius bone in the elbow of a patient includes a head and a curved stem which extends therefrom. Specifically, the head is shaped for simultaneous articulation with both the capitellum of the humerus bone and with the radial notch of the ulna. Also, the stem is curved, or arcuate shaped, in order to accommodate the anatomy of the proximal radius after resection of the radial head, and to securely anchor the prosthesis when the stem is inserted into the medullary canal of the radius bone.
In detail, the head of the prosthesis of the present invention has a proximal surface that is formed as a substantially concave recess, and it has a distal surface that is formed with a hole. Preferably, the proximal surface of the head is made of highly polished cobalt chrome or a ceramic material to facilitate articulation of the head with the capitellum and with the radial notch of the ulna bone. More specifically, as envisioned for the present invention this articulation involves a relative sliding motion between the head of the prosthesis and both the capitellum and the radial notch of the ulna.
The stem of the prosthesis has a proximal end and a distal end. Also, the stem is preferably made of a material such as cobalt chrome or titanium. Further, it has an extension at its proximal end that is slightly tapered with a proximally decreasing cross section to facilitate insertion of the extension into the hole in the distal surface of the head. Preferably, the extension is configured as a morse taper. In any event, as intended for the present invention, the proximally decreasing cross section of the extension establishes an interference fit between the extension of the stem and the head that helps to hold the head on the stem.
As indicated above, the stem of the prosthesis is curved in shape and is generally arcuate. Importantly, the configuration of the stem is intended to structurally mimic the shape of the medullary canal of the radius bone. In general, to accomplish this the stem is formed with a first portion and a second portion. Specifically, the first portion is attached to the head and extends distally therefrom substantially along a first axis. The second portion then extends distally from the first portion substantially along a second axis. In their relation to each other, the first axis and the second axis define an angle xcex1 therebetween. Preferably, the angle xcex1 will be in a range of about 5 degrees to approximately 25 degrees. The curve of the stem that is established by the angle xcex1 can be achieved using a radius of curvature that is in a range of about 0.5 inch to approximately 3 inches. Further, as it extends distally along the first and second axes, the stem is tapered with a distally decreasing cross section. This facilitates insertion of the stem into the medullary canal of the radius bone and helps to anchor the prosthesis on the radius bone. The stem also has a roughened or textured surface, which interacts with the medullary canal of the radius bone to help hold the stem on the radius. For purposes of the present invention, the surface of the stem can be coated with a material such as titanium, cobalt-chrome beads or hydroxyapatite. In addition to these structural features, the stem also has a collar that is formed on the first portion of the stem. As intended for the present invention, this collar is positioned to limit insertion of the stem into the medullary canal.
In order to implant the prosthesis of the present invention into the radius bone of a patient, it is first necessary to do a resection of the radius by removing the radial head and exposing the medullary canal. When doing this, it is important that the prosthesis have the same functional dimensions as the amount of bone that is to be removed. For the present invention, the required accuracy for this exchange is achieved by using a resection guide.
In accordance with the present invention, the resection guide includes an alignment rod which has a series of notches on its surface near the proximal end. The resection guide also includes an adjustable flange and a reference flange. Specifically, the reference flange is pre-positioned at a fixed location between the distal and proximal ends of the alignment rod. When the adjustable flange is engaged with the alignment rod, it can be selectively fixed at one of the notches on the alignment rod.
As intended for the present invention, the notches on the alignment rod are located relative to the reference flange so that, when the adjustable flange is affixed to the alignment rod, the distance between the adjustable flange and the reference flange will correspond with a particular head size of the prosthesis. Thus, upon the engagement of the adjustable flange with a notch, the size of the head of the prosthesis can be determined.
For the operation of the resection guide, after an incision has been made to establish access into the elbow of the patient, the adjustable flange is engaged with the proximal end of the alignment rod. The adjustable flange is then positioned against the articular surface of the capitellum. Also, the distal end of the alignment rod is positioned over the ulna styloid process. When so positioned, the alignment rod establishes the axis of rotation of the radius about the ulna. Stated differently, the adjustable flange is oriented to align the alignment rod with the center of rotation of the capitellum and with the styloid of the ulna. Next, once the alignment rod is properly positioned between the capitellum and the styloid of the ulna, the alignment rod and the reference flange that is attached to the alignment rod are guided proximally or distally relative to the adjustable flange until the adjustable flange is located at a notch on the alignment rod and the reference flange is positioned at a desired location on the radius. At this point, the reference flange is positioned to establish a distance between the adjustable flange and the reference flange that is equal to a desired length for radial head resection.
As the alignment rod is guided to position the reference flange at a desired location on the radius for resection, the distal end of the alignment rod is simultaneously positioned at the styloid process of the ulna. Thus, the forearm of the patient is positioned for an osteotomy in a plane that is substantially perpendicular to the axis of rotation of the radius bone about the ulna. In this position, the reference flange can be used as a guide to establish a cutting line for the resection. Specifically, the reference flange will guide the cutting of the radius in a plane that will be perpendicular to the axis of rotation of the radius bone about the ulna. The cut thus made to the radius bone will facilitate insertion of the curved stem of the prosthesis into the medullary canal of the radius bone.
Once the radial head has been removed, it can be replaced by the prosthesis. Specifically, this is done by inserting the stem of the prosthesis into the medullary canal and positioning the head of the prosthesis on the stem for articulation with the capitellum and the ulnar notch.